Vacuum free 4-cycle?

[DKJones96]

Does anybody here know if someone is working on an engine that has no manifold vacuum?

I have a few things i've gome up with that involve an IC engine with no manifold or chamber vacuum at part throttle. The engine basicly purges the charge from the cylinder again into the intake manifold(against a reed valve to create a bit of pressure to force exhaust out on overlap) until it reaches your 'throttle' point. My way of doing it is infinately variable and completely mechanical and to help with this I've developed a completely mechanical way to have variable compression ranging say from 8-12:1 and also thought of a low pressure direct injection system for it. I plan on building this for a Toyota 5M-GE I6 engine starting in a couple weeks.

I was just wondering what you guys thought about efficiency increases and if anyone is working on this kind of stuff. Sadly...I'm only 22 so most people don't take me seriously. Just wondering what you guy thought.

 

[Fabrico]

No manifold vacuum? So what makes fresh air want to rush into the cylinder?

 

[BrianPetersen]

Diesel engines don't have (significant) intake manifold vacuum because they are unthrottled, and most of them these days are turbocharged.

BMW's Valvetronic system operates not entirely unlike what you are describing, except that they are shortening intake lift and duration at part load so that the air never goes into the cylinder in the first place, rather than sucking it in and then pushing it out again. It normally operates unthrottled (i.e. no significant intake vacuum), although there is a throttle only used as an emergency back-up in case the Valvetronic system fails.

Honda's upcoming A-VTEC system will also operate conceptually like this, albeit with a different mechanical arrangement.

These systems are reported to improve part-load fuel consumption about 10 - 15 percent.

Variable compression ratio is ordinarily extremely difficult to do in a way that doesn't create more problems than it solves (generally it screws up the shape of the combustion chamber when it is in low-compression mode) but if you have found some way around that, go nuts with it.

Direct injection systems ordinarily need to operate at a rather *high* pressure in order to get adequate atomization of the fuel. Gasoline-direct-injection systems use a higher pressure than normal outside-the-cylinder systems, but lower than common-rail diesel injection systems.

 

[patprimmer]

BMWs Valvetronic effectively uses the valve as a throttle, so from a pumping losses point of view it is still throttled.

I think the OP is talking about no change in restriction to airflow into the cylinder at all.

It is not clear to me how power delivery will be controlled.

There will of course be some vacuum at times as the system will not be 100% VE at all times.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tmoose]

the A-F ratio needs to be between something like 8-1 and 18-1 to be combustible by normal means (spark plug). At low and part throttle that means there must be a zone of richness near the spark lug to initiate combustion that can then progress to the leaner portions of the chamber. Stratified charge.

These guys have been working on igniting lean mixtures for 30 years, and report they are now up (down) to 30-1 ratios.

http://www.leanburnignition.com/

 

[DKJones96]

The throttle is controlled by closing the intake valve later and later. Like idle might be closing the intake valve 45 degrees BTDC on the compression stroke(never been done so i just have to throw a number out there.

So instead of working against a strong vacuum at idle it pulls a full charge then purges most of it again til it gets to the 'displacement' it needs. It should not only save fuel but smooth out idle greatly on smaller 4 cylinder engines.

 

[Fabrico]

Varying engine breathing with valve timing is not new. Using reed valves on 4-strokes is not new. So, what's new?

 

[turbocohen]

If a plasma torch ignition system with enough btu's available to light lean gasoline mixtures then you can unthrottle an SI engine. It has been donie in the lab with over 600MJ and that is enough to fry your weiner if current reaches your body.

Yes it is doable, no it is not practical yet.

 

[hydrae]

it is called the atkin(sp) or miller cycle, they are used in the toyota hybrids (maybe others). but they do not run well at low rpm's. hence the use in hydbrids.
Hydrae

 

[GregLocock]

There is no reaosn why an Atkinson cycle would not run at low speeds, and to be honest I can't see any particular reason why a Miller wouldn't.

Both over-expanded engines have idle speeds of around the same as that used in Otto engines of the same capacity.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dicer]

Like Brian says diesel, though some older ones did throttle the air intake.

And like also mentioned there is always going to be a vaccum, as long as you have a piston in a cylinder drawing in the air. Then add an airfilter etc. Of course unless you are boosting with positive inlet pressure.

 

[DKJones96]

I understand that there will be a little bit of a vacuum. I guess i was too vauge in either my description and title or i had just assumed that everyone would understand that the only vacuum that won't be there is the one created in the intake manifold for engine throttling. I guess the closest thing to the engine itself would be a miller cycle engine except this one can keep the intake valve open way past what miller cycle does for 'throttling' and those usually have the cam set to the close angle via cam grind. This valvetrain i'm going to build will give you the same cam profile at WOT that any engine nowadays does.

 

[Andreb]

Why pull the extra air into the cylinder if you are just going to push it back out again before closing the intake valve. Close the intake valve on the intake stroke as soon as the required amount of air is in the cylinder.

I think I read something on some really early gas burning engines that did just that, I think they had separate valves for the air and gas. They used valve gear something like the Corliss steam engines and avoided the throttle loss in the same way as the Corliss, it takes work to push/pull a gas thru a restricted opening.

Of coarse at 50 RPM a short valve open close event is not all that short compared to what you need at 5000 RPM.

 

[DKJones96]

Because it takes less power to pump the air twice than it does to work against the vacuum. Especially the larger the engines displacement is.

 

[Lou Scannon]

Intuitively that seems wrong. The vacuum will act like a spring, assuming the cylinder sealing is good. Pumping air back & forth through the open valve is wasted work.

 

[turbocohen]

One of the reasons a diesel is more efficient it the low pumping loss. Pumping a vacuum takes work. Lean burn is not efficient combustion but the reason it yeilds efficiency gains is the reduced pumping against atmosphere.. Modern electronic throttled/evt engines with 6speed transmisions and better calibrations lug the engine stochiometric at a lower speed so manifold pressures are closer to atmospheric and trapped volume has more tumble introduced to reduce burn time.. less delta P across the piston = less work and lower rpms = less moting friction.
Nowadays some truck calibrations use tricky fuel cutoff or decel fuel and throttle to enhance engine braking while keeping the cat lit but thats another story...

 

[patprimmer]

Diesels also have about double the expansion ratio to a petrol engine.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

Heywood has a nice plot of the efficiency gains from increasing the expansion ratio (eg via Atkinson). That last litle bit doesn't really do you much good, much better to use it to drive an expansion turbine.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[globi5]

...an expansion turbine driving a compressor, allowing to increase the power output. Therefore decreasing the size of the engine and thus reducing the pumping losses (less displacement = smaller pumping losses). And of course a smaller engine also needs to be driven at higher throttle settings in order to move the same vehicle (= less 'pumping loss time').
Besides weight reduction possibly another reason why turbocharged engines are becoming increasingly popular.

Then again Toyota does use the atkinson/miller cycle engine on its Hybrids - so there's some benefit to it.

One could question why there is currently no miller cycle engine or very few engines with compressors on the market?
(I guess a turbo is small, few moving parts uses an inherently efficient compressor, where its rpm is not fixed to the rpm of the engine its actually not that surprising even considering its lag).

Regarding Diesel: Keep in mind Diesel-fuel is also 15% more dense than gasoline (also adding to the fuel economy of a diesel engine - a factor often forgotten when a Hybrid is compared to a Diesel).

Back to the topic, to what level does EGR and valve overlap reduce pumping losses on a modern engine?

 

[Fabrico]

Have to agree with hemi on this one.
As mentioned before...block a fan or vacuum cleaner, and hear it immediatley speed up.